This invention relates to a process for controlling a grinding installation comprising a roller mill in which the grinding force is hydraulically generated and the ground material is pneumatically discharged, a separator, a recirculating air fan and a storage silo for the ground material, and wherein:
(a) the mill is operated under a negative gas pressure kept constant by a first automatic control system, PA1 (b) the differential pressure between the gas inlet end of the mill and the gas outlet end of the separator is kept constant by a second automatic control system which alters the amount of material delivered to the mill, and PA1 (c) the gas temperature prevailing at the gas outlet end of the separator is kept constant by a third automatic control system.
It is known that a grinding installation may be controlled by a process of the type described above. In this connection, the value known as the differential mill pressure (which is kept constant by the second automatic control system) is a measure of the amount of material in the grinding compartment of the mill and in the following gravity air separator. However, this second automatic control system is only effective if at the same time the amount of gas flowing through the mill and hence the gas velocity are kept constant. This is the responsibility of the first and third automatic control systems because the output of the fan depends upon the static pressure increase and upon the gas temperature for a constant rotational speed and a constant position of the guide apparatus.
This known process is based on the use of three automatic control systems and ensures constant mill operation for a certain mill throughput. Any variations in the amount of ground material required by the consumer have to be absorbed by the storage silo for the ground material (end product). On the one hand, this limits the permitted variations in consumption for a constant mill output and, in addition, gives rise to the further disadvantage that the storage silo for the end product may have to be operated for prolonged periods at a low filling level which, in the case of a storage silo for coal dust for example, is undesirable in the interests of explosion prevention.